The present invention relates to a signal processing circuit for demodulating amplitude-modulated signals as are, for example, present in RFID systems.
Amplitude shift keying (ASK) is a digital type of modulation which has found various fields of application due to its low-expense signal processing. Exemplarily, it is employed in RFID systems (RFID=radio frequency identification) allowing wireless identification of a transponder and wireless data transmissions. Further fields of application of amplitude shift keying are, for example, in the field of radio clocks where a carrier signal transmits current time and datum information to synchronize time. Radio beacons for identifying a location are another example. Here, a carrier signal is modulated by a continuous tone in the audio range for an easier identification, the audio tone in turn is modulated (sampled) according to a desired Morse code so that both the direction can be determined and the transmitter itself can be identified using the overall signal aimed at. The so-called on-off keying (OOK) may be mentioned as the simplest variation of amplitude shift keying. In this method, a carrier signal is switched on and off to transmit a binary “1” and “0”, respectively.